As set out in particular in U.S. Pat. No. 5,322,415, it is advantageous to be able to fold the blades of a helicopter rotor in order to reduce the space occupied thereby.
The blocking device described in that document comprises a bracket secured to the rotor hub via tines for securing a damper, the bracket being installed on a permanent basis; the device also includes a second bracket that is secured temporarily to the first bracket and to the pitch lever of the blade that is to be prevented from moving, with this being done via three pins; the drawback of that system is that the permanent bracket increases the weight of the rotor hub quite significantly.
French patent No. 2 750 948 also describes a pitch-blocking device in which blocking is obtained by mutual engagement between a finger and a fork, one of those elements being secured to the hub and the other element being secured to a blade; that system includes locking/unlocking actuators, and it is complex.
French patent Nos. 2 754 779 and 2 765 550 describe other systems for blocking foldable blades: the first document describes a device comprising a plurality of blocking rods for preventing relative movement between the hub and a rotary swashplate for driving the pitch control rods; for this purpose, the ball-jointed top end of each blocking rod is secured to a plate of a part connecting a lag damper to the hub; the second document describes a device comprising blocking rods for preventing movement of the non-rotary plate of the pitch control system.
Those systems are relatively complex to use.